Arduino temperature sensor

Question

I am trying to build a small program with arduino using a temperature-sensor.

I thought I knew how to do it but I'm getting some weird outputs.

Here is my code:

int sensorPin = 0;
void setup()
{
    Serial.begin(9600);
}
void loop()
{
    int reading = analogRead(sensorPin);
    float voltage = reading * 5.0 / 1024;
    float temperatureC = (voltage - 0.5) * 100;
    Serial.print(temperatureC); Serial.print(" degrees C, ");
    Serial.print(voltage); Serial.println(" volts");
    delay(1000);
}

This code gives me the output:

-26.56 degrees C, 0.23 volts
-26.56 degrees C, 0.23 volts
-27.05 degrees C, 0.23 volts
-26.56 degrees C, 0.23 volts
-26.07 degrees C, 0.24 volts
-26.07 degrees C, 0.24 volts

Why is it - in degrees? and Why can I change it to any pin I want and it will still give me a similar output?

Answer 1

Analog input 0 is not pin 0.

You should use the defined symbols:
A0,A1,...,A7
for analog inputs.

Try

int sensorPin = A0;  

and your program should work.

If you are curious about the actual values, under your Arduino IDE install look in the file
..\hardware\arduino\variants\standard\pins_arduino.h

Answer 2

You are reading this input correctly. In order for you to not get negative degrees, you'll have to process it differently.

With this:

float temperatureC = (voltage - 0.5) * 100;

Any values < 0.5 result in multiplying a negative number by 100.

Try breaking this down using commutative property.

• (voltage - 0.5) * 100 is the same as (voltage * 100) - (0.5 * 100).
• This can be further simplified to (voltage * 100) - 50.

Still, for all values where voltage < 0.5 the temperature will be negative.

• I would suggest multiplying temperatureC by -1 to make it positive and not putting the sensor near anything that is `~ >= 50 degrees Celsius.

Also, As jdr5ca pointed out here, you're not actually getting any data from the sensor yet... :(

You are probably getting noise (or garbage) from whatever pin0 is.

EDIT

It is best practice to use parentheses to make order of operations more clear.

• ie:
  • float voltage = reading * 5.0 / 1024;
  • should be
  • float voltage = reading * (5.0 / 1024);

Answer 3

First, you have to use the defined symbols A0

int sensorPin = A0;

in the next

float voltage = reading * (5.0 / 1024);

There is an example in File/Examples/Basic/AnalogReadSerial